Various means and methods have heretofore been proposed for preparing castings, and molds therefore, using the so-called "lost wax" method. Use of the lost wax method within the dental profession for the purpose of casting crowns and other such dental restorations is similarly well known. Of particular note with regard to dental castings is the fact that dental castings require extreme accuracy for the final product to be both useful and comfortable. Heretofore, metallurgical characteristics have virtually dictated that the castings be prepared from precious metals such as gold. Numerous attempts have been made at preparing dental castings from base metal alloys but experience has shown that base metal castings generally require excessive finish work to allow the restoration to fit properly.
Another problem inherent in these casting techniques utilizing the lost wax method is the fact that curing of the investment material is an exothermic reaction and as the investment material hardens, heat is generated. This heat then causes "swelling" of the investment material which, when using conventional rigid casting rings and sprue formers, results in at least some compression on and distortion of the wax model within the investment. Any degree of compression against the wax model while preparing a dental restoration is unacceptable because it will distort the restoration pattern and ultimately impair the fit of a casting produced therefrom. Thus, when the pattern is burned out and molten metal is introduced into the void formed within the hardened investment material, the casting thereby created will not correspond exactly to the original pattern. This results not only in discomfort to the patient for whom the restoration has been prepared, but unnecessary expenditure of time, energy and money for the patient, the dentist, and the dental technician to modify the dimensions of the casting so that it will fit reasonably properly.
The problem of obtaining accurately dimensioned castings is further magnified when utilizing base metal alloys wherein the only effective solution heretofore available required detailed and expensive hand finishing of the castings or the taking of completely new impressions and making new castings until a satisfactory result is obtained. An unacceptable alternative required the patient to accept an inferior final product. It is toward the resolution of these prior art problems in producing accurately dimensioned castings in an economically viable manner that the present invention is directed.